In the field of internal combustion engines, it is known to provide an oil separation device in a blow-by passage for returning the blow-by gas in a crankcase chamber back to an intake system. For instance, in the arrangements disclosed in Patent Documents 1 and 2, a head cover formed by combining a plurality of members is internally provided with a gas liquid separation chamber in which a plurality of baffle plates are positioned. The blow-by gas containing oil mist changes the flow direction thereof by colliding with the baffle plates as the blow-by gas travels from the inlet to the outlet of the gas liquid separation chamber. When the blow-by gas changes the flow direction thereof, the oil contained in the blow-by gas is caused to adhere to the baffle plates owing to the inertia thereof, and is thereby removed from the blow-by gas.